1. Field of the Invention
The present invention relates to a wireless communication method and a wireless communication device.
2. Description of the Related Art
A receiver of a Personal Handyphone System (PHS system) as one example of a single-carrier communication system has the configuration shown in FIG. 1, and includes an antenna 101, a radio circuit 102, a local oscillator 103, an analog to digital (A/D) converter 104, and a digital demodulation circuit 105. The radio circuit 102 provided with a filter has a bandwidth of 300 kHz. The local oscillator 103 gives a signal having a frequency of f1. The A/D converter 104 performs an A/D conversion on a received baseband signal detected and extracted by the radio circuit 102. The digital demodulation circuit 105 demodulates the received digital signal converted by the A/D converter 104 to output an original transmitted signal with f1.
However, in the single-carrier communication system, the communication speed is slow, and is strongly desired to be increased. A communication system responding to increase in the communication speed is a multicarrier communication system. The multicarrier communication system is used as one scheme for increasing a communication speed in the field of wireless communication.
A description will be given of an example of a multicarrier communication system to which a PHS system is applied. In a PHS system, the width of one carrier is set 300 kHz (symbol rate is 192 kHz) because of standardization. To increase the communication speed with the width of one carrier maintained, there is basically no other way but employing multilevel transmission. For example, as a scheme for increasing the communication speed, the expansion of the carrier width (several megahertz in a 3rd generation (3G) system) is effective. However, to achieve this, the standards need to be changed. In that case, such a system becomes totally different from a current PHS wireless device.
Accordingly, to widen the bandwidth with the standards maintained and increase the communication speed, a PHS receiver having the configuration of FIG. 2 is also conceived. The PHS receiver has a configuration in which receiving units R-UN1 and R-UN2 using two carriers are mounted. However, this is, so to speak, a configuration in which two single-carrier receivers shown in FIG. 1 are incorporated in one casing. In particular, the size of the casing is increased due to two channels of radio circuits 102 being mounted, and power consumption is also increased. Thus, this is unrealistic.
With the recent advancement in technology, even in the case where digital signal processing is very complicated, the load thereof can be made smaller in terms of size, power consumption, and the like compared to that of a radio frequency (RF) circuit, and demultiplexing is becoming capable by digital signal processing. Accordingly, as a multicarrier receiver which receives two carriers simultaneously, one having the configuration shown in FIG. 3 is realistic. This multicarrier receiver has a configuration in which the bandwidth is seemingly widened by mounting a radio circuit 102 having a filter with a width of 600 kHz that allows the passage of two adjacent signals each having a bandwidth of 300 kHz, and in which demultiplexing into signals each having a bandwidth of 300 kHz as heretofore is performed by digital signal processing using two channels of digital filters (DF) 106-1 and 106-2 and digital demodulation circuits 105-1 and 105-2 to increase the communication speed. In this patent application, a processing scheme in which the bandwidth of a radio circuit is widened and in which demultiplexing is performed by digital signal processing is referred to as multicarrier processing.
Here, multicarrier processing does not need to be performed at both a transmitter and a receiver. The transmitter needs only to have the function of transmitting a plurality of carriers. In an example of a PHS system, base stations are trending toward multichannelization and, in many cases, originally have the function of transmitting a plurality of carriers. Accordingly, a multicarrier communication system in which the communication speed is increased by utilizing a plurality of RFs of base stations can be realized by merely installing a multicarrier processing function on mobile terminals.
However, such multicarrier communication has the following technical problem to be solved. In the case where multicarrier communication is performed, if the difference between the levels of signals f1 and f2 is large, there are cases where demultiplexing cannot be appropriately performed by digital signal processing at the receiving end due to the problem of the dynamic range of an A/D converter. For example, a 10 dB difference between the levels of the received signals f1 and f2 corresponds to a 10-fold difference in power (3.16-fold difference in amplitude), and a 20 dB difference between the levels thereof corresponds to a 100-fold difference in power (10-fold difference in amplitude). In the case where the level difference is 20 dB and the A/D converter is 12 bits, a larger signal has a range of up to 2048, but a smaller signal has a range of merely up to 204. Thus, quantization error becomes large, and this can cause deterioration in reception performance.